some by way of Cuba, others by way of Central America, where a few may linger, the main body presses onward beyond the Amazon into central and southeastern Brazil. On the return journey they reach the southern border of the United States in March and April.
The catbird is found in summer throughout the eastern United States and British Provinces, and in winter in the southern States, Cuba and Middle America to Panama. Our common robin is very erratic in habits of migration. Occasionally a few may winter in dense swamps as far north as southern Canada and Maine, but the majority spend the winter in the Southern States. The chimney swift is found in summer in eastern North America and thence north to Labrador and the fur countries. The winter is spent to the south of the United States. Cliff and barn swallows, which are found over nearly all North America in summer, may penetrate to Brazil, Paraguay and the West Indies in winter. The scarlet tanager passes the winter in the West Indies, Central America and northern South America, and the familiar indigo bird may go as far as Veragua.
The great group of warblers, of which some 70 species are found in the United States, has been mentioned before. They are all strongly migratory and mainly pass beyond our southern borders in winter, although a few individuals of a single species—the yellow-rumped warbler—have been known to winter on Cape Cod. Some of them visit the West Indies but the larger number, after rearing their young in the dense coniferous forests of the Hudson's Bay region or even in Alaska, spend the winter in Mexico, Central America or northern South America.
The sparrows as a group are also strictly migratory. Quite a number, such as the tree sparrow (monticola), snowflake (Plectrophenax hyperhoreus) and longer spur (Calcarius lapponicus) breed far to the north of the United States in Arctic districts, and come down in winter into the northern states or irregularly farther south. Many species which breed mainly north of the United States only go into the middle and southern states during the winter, while a few may reach the West Indies, Mexico, Central America or northern South America.
But after having described these migration routes and the wonderful journeys over continents and vast oceans, the mystery of mysteries—How is it possible for the birds to find their way so unerringly?—still remains without a wholly satisfactory answer. As in the case of theories propounded to account for the origin of migration, so numerous suggestions have been made to explain this wonderful faculty. Thus Dr. Von Middendorff, a distinguished naturalist who studied exhaustively the migrations in the Russian Empire, suggests that because all the spring movements in that country are toward the